The present invention relates to a steering system for an automotive vehicle and, more particularly, to a rear wheel steering sub-system having fail-safe capability.
In modern automotive vehicles having two front wheels and two rear wheels, the steering system incorporated therein sometimes includes a rear wheel steering sub-system in addition to a front wheel steering sub-system. By additionally including such a rear wheel steering sub-system, both the navigation of road turns and the execution of parking maneuvers can generally be executed more efficiently.
In modern automotive vehicles incorporating such a steering system, the front wheel steering sub-system typically includes a rotatable handwheel (commonly referred to as a xe2x80x9csteering wheelxe2x80x9d) that is used by the vehicle operator or driver to steer the vehicle. This handwheel is linked to one end of a rotatable steering shaft. The other end of the rotatable steering shaft is typically linked to a front transmission mechanism. The front transmission mechanism may include, for example, a rack-and-pinion gear mechanism or a worm gear mechanism that converts any rotational movement of the handwheel and steering shaft into a back-and-forth linear motion that is applied via left and right front linkage assemblies to the two front wheels of the automotive vehicle. With such a configuration, a particular rotational movement of the handwheel is converted into a corresponding linear motion that is applied via the front linkage assemblies to the two front wheels for setting the two front wheels at a corresponding steering angle. In this way, the driver can manually apply torque to the handwheel and orient the handwheel in various desired rotated positions to thereby steer the two front wheels of the automotive vehicle at various desired corresponding steering angles.
The rear wheel steering sub-system in such a steering system typically includes, first of all, an electronic controller or control unit (ECU). The ECU is an expensive and complex unit that is program-controlled by software programs stored in a memory within and/or associated with the ECU. In addition to the ECU, the rear wheel steering sub-system also typically includes a vehicle speed sensor, at least one sensor for sensing the steering angle of the two front wheels, at least one sensor for sensing the steering angle of the two rear wheels, and an actuator which are all electrically connected to the ECU. The vehicle speed sensor serves to sense the speed of the automotive vehicle, and the actuator serves to mechanically actuate the rear wheel steering sub-system to set the two rear wheels at a determined steering angle. In such a configuration, the ECU receives electric signals from the vehicle speed sensor, each front wheels steering angle sensor, and each rear wheels steering angle sensor to compute and determine a safe and appropriate steering angle for the two rear wheels. Once an appropriate steering angle is determined in this way, the ECU electrically communicates and supplies an appropriate level of electric current to the actuator to thereby electrically empower the actuator, with the help of a rear transmission mechanism and left and right rear linkage assemblies, to mechanically situate the two rear wheels at the steering angle determined by the ECU.
Given such a rear wheel steering sub-system, if the ECU begins to malfunction, for example, due to a software xe2x80x9cglitchxe2x80x9d or the failure to detect and compensate for a malfunctioning sensor, the ECU may inadvertently dictate that the actuator steer the two rear wheels at a sharp steering angle even when the automotive vehicle is traveling at a high rate of speed. In such a situation, the vehicle may suddenly become uncontrollable.
To prevent such a situation from occurring, some automotive vehicle manufacturers have adopted a fail-safe strategy used by the aerospace industry in many modern aircraft and have included a similar strategy within the designs of the rear wheel steering sub-systems incorporated within their vehicles. In particular, the potential adverse effects of a malfunction in a critical system within a modern aircraft are guarded against by a cross-checking fail-safe strategy that requires the inclusion of redundant hardware, redundant and/or multiple electronic controllers, and safety critical software with electric signal redundancy within the aircraft design. Such a fail-safe strategy requiring redundant hardware and/or software with electric signal redundancy has been analogously implemented by some vehicle manufacturers within the designs of the rear wheel steering sub-systems incorporated within their vehicles. In general, however, such redundancy undesirably increases both the overall design complexity and the cost of manufacturing such rear wheel steering sub-systems and incorporating such sub-systems within automotive vehicles.
In light of the above, there is a present need in the art for a steering system including a rear wheel steering sub-system with fail-safe capability that has both a relatively simple design and an economical manufacturing cost.
The present invention provides a steering system for an automotive vehicle having two front wheels and two rear wheels. According to the present invention, the steering system basically includes, first of all, a speed sensor for sensing the speed of the vehicle, means for steering the two front wheels at a desired steering angle, one or more steering angle sensors for sensing the steering angle of the two front wheels, and an axially displaceable rear rack. The rear rack is linked between the two rear wheels and serves to steer the two rear wheels at a determined steering angle. In addition, the steering system also basically includes a centering resilient member extending along the length of the rack, a rear transmission mechanism connected to the rear rack, and an actuator connected to the rear transmission mechanism. The centering resilient member has a resiliency enabling the rear rack to return the two rear wheels to a neutral steering angle position. The actuator serves to axially displace the rear rack via the rear transmission mechanism against the resiliency of the centering resilient member. Furthermore, the steering system also basically includes one or more steering angle sensors for sensing the steering angle of the two rear wheels, an electronic control unit, and an electronic power regulation unit. The electronic control unit serves to determine a steering angle for the two rear wheels from electric signals received from the vehicle speed sensor, each front wheels steering angle sensor, and each rear wheels steering angle sensor. After doing so, the electronic control unit then supplies an appropriate level of electric current to the actuator to thereby electrically empower the actuator to steer the two rear wheels at the determined steering angle. The electronic power regulation unit, on the other hand, serves to selectively and electrically disable the actuator according to electric signals received from the vehicle speed sensor, the level of the electric current supplied by the electronic control unit to the actuator, and a predetermined electric current limiting function.
In a preferred embodiment of the present invention, the front wheels steering means includes a rack-and-pinion gear front transmission mechanism, and the centering resilient member is a coil-type spring. In addition, the rear transmission mechanism is preferably a ball screw mechanism, and the actuator preferably includes a brushless, direct-current (DC) electric motor. Furthermore, the steering system preferably further includes a yaw rate sensor, a lateral acceleration sensor, sensors for sensing the rotational speeds of the two front wheels, and sensors for sensing the rotational speeds of the two rear wheels. The yaw rate sensor, the lateral acceleration sensor, the front wheels speed sensors, and the rear wheels speed sensors are preferably electrically connected to the electronic control unit such that the electronic control unit is capable of receiving electric signals therefrom and determining a safe and appropriate steering angle for the two rear wheels.
Also, in a preferred embodiment of the present invention, the electronic power regulation unit preferably includes an electronic comparator and an electronic controller. The electronic comparator is preferably electrically connected between the electronic control unit and the actuator. The electronic controller, on the other hand, is preferably electrically connected between the vehicle speed sensor and the electronic comparator. In such a configuration, the electronic controller is capable of receiving electric signals from the vehicle speed sensor, determining an electric current limit level according to the electric signals received from the vehicle speed sensor and the predetermined electric current limiting function, and transmitting an electric signal representative of the value of the electric current limit level to the electronic comparator. Also, in such a configuration, the electronic comparator is capable of receiving an electric signal representative of the value of the level of electric current supplied by the electronic control unit to the actuator, receiving the electric signal representative of the value of the electric current limit level, comparing the value of the level of the electric current and the value of the electric current limit level, and electrically disabling the actuator only when the value of the level of the electric current is greater than the value of the electric current limit level.
In general, the centering resilient member and the electronic power regulation unit together provide the steering system according to the present invention with rear wheel steering fail-safe capability with little or no hardware and software redundancy. As a result, the overall steering system according to the present invention has both a relatively simple design and an economical manufacturing cost. Furthermore, other advantages, design considerations, and applications of the present invention will become apparent to those skilled in the art when the detailed description of the best mode contemplated for practicing the invention, as set forth hereinbelow, is read in conjunction with the following drawings.